Process for preparation of rare earth silicate phosphor

ABSTRACT

A rare earth element-activated rare earth silicate phosphor is prepared by the steps of (1) heating a rare earth carboxylate with an alkoxyalcohol to obtain a solution; (2) adding to the solution a silicon alkoxide and a compound of the element A, to prepare a mixture; and (3) subjecting the mixture to thermal decomposition.

FIELD OF THE INVENTION

The invention relates to a new process for preparation of a rare earthsilicate phosphor.

BACKGROUND OF THE INVENTION

A rare earth silicate (Ln₂SiO₅:A in which Ln is Y, Gd and/or Lu; and Ais Ce, Sm, Eu, Tb and/or Zr) is known as a stimulable phosphor [forexample, as described in J. Phys. D: Appl. Phys., vol. 24 (1991), pp.997-1002]. When the stimulable phosphor is exposed to radiation orultraviolet rays (primary excitation), the phosphor absorbs and stores aportion of the energy of radiation or ultraviolet rays. The stimulablephosphor then emits stimulated light in the visible wavelength regionwhen exposed to electromagnetic wave such as visible light or infraredrays (secondary excitation). The stimulable phosphor is practicallyutilized in a radiation image recording and reproducing method, andJP-2,00,696A and JP-3,290,497B propose use of the rare earth silicate inthe method as a phosphor of a radiation image storage panel (which isalso referred to as “imaging plate”).

The radiation image recording and reproducing method employs a radiationimage storage panel containing an energy-storing phosphor such as thestimulable phosphor. A typical process of the method comprises the stepsof causing the stimulable phosphor of the storage panel to absorbradiation energy having passed through an object or having radiated froman object; sequentially exciting the phosphor with a stimulating light(such as a laser beam) to emit stimulated light; and photoelectricallydetecting the emitted light to obtain electric signals giving a visibleradiation image. The storage panel thus processed is subjected to a stepfor erasing radiation energy remaining therein, and then stored for theuse in the next recording and reproducing procedure. Thus, the radiationimage storage panel can be repeatedly used.

JP-2001-255,610A discloses a radiation image forming method, which is analternative process of the radiation image recording and reproducingmethod. The stimulable phosphor of the storage panel used in theconventional recording and reproducing process plays both roles ofradiation-absorbing function and energy-storing function. However, thosetwo functions can be separated in the process. In the process, aradiation image storage panel comprising the stimulable phosphor (whichstores radiation energy) is used in combination with a phosphor screencomprising a different phosphor which absorbs radiation andspontaneously emits ultraviolet or visible light. This process comprisesthe steps of causing the radiation-absorbing phosphor of the phosphorscreen to absorb and convert the radiation having passed through anobject or having radiated from an object into ultraviolet or visiblelight; causing the energy-storing phosphor (i.e., stimulable phosphor)of the storage panel to store the energy of the converted light asradiation image information; sequentially exciting the stimulablephosphor with a stimulating ray to emit stimulated light; andphotoelectrically detecting the emitted light to obtain electricsignals, whereby giving a visible radiation image.

In the radiation image recording and reproducing method (and in theradiation image forming method), it is required to obtain a clear imagewith a small dose of radiation. In consideration of this requirement,the radiation image storage panel (or the phosphor screen) preferablyhas a phosphor layer in which the phosphor is so densely packed that theradiation is efficiently absorbed. Accordingly, the phosphor preferablyhas a high true density.

It is known that lutetium silicate has a high true density (7.4 g/cm²)and that a silicate phosphor comprising lutetium has a high meltingpoint (higher than 2,000° C.). Accordingly, the silicate phosphor hasbeen hitherto prepared by the steps of melting starting materials at avery high temperature (above 2,000° C.) and gradually drawing up asingle crystal of the phosphor from the melt. It is, therefore, not easyto prepare the lutetium silicate phosphor, and hence it is desired thata rare earth silicate phosphor having a high true density, particularlya silicate phosphor containing a heavy rare earth such as lutetium, bemore easily prepared.

The rare earth silicate phosphor is also known to absorb radiation suchas X-rays and instantly emit a light in the visible wavelength region(instant emission). Because of this property, it is suggested in NuclearInstruments and Methods in Physics Research A, vol. 416(1998), pp. 333;IEEE Transaction on Nuclear Science, vol. 41(1994), No. 4, pp. 689; andibid., vol. 47(2000), No. 6, pp. 1781, that the rare earth silicatephosphor be utilized as a scintillator, which is required to comprisephosphor packed densely enough to absorb radiation efficiently.

J. Phys. D: Appl. Phys., vol. 24 (1991), pp. 997-1002 describespreparation of Y₂SiO₅: (Ce, Sm) phosphor. The disclosed phosphor isprepared by a solid phase reaction method which comprises the steps ofmixing Y₂O₃, CeO₂, Sm₂O₃, SiO₂ and NH₄ (flux) and firing the mixture.

JP-2-300,696A discloses a stimulable phosphor represented by theformula: Y_(x)Lu_(y)Gd_(z)SiO₅:aA,bB in which x, y and z are numberssatisfying the conditions of x+y+z=2, 0<x, 0≦y, 0≦z; A is Ce and/or Tb;B is Zr and/or Sm; and a and b are numbers satisfying the conditions of2×10⁻⁵<a<0.02 and 2×10⁻⁵<b<0.02, respectively. The disclosed phosphor isprepared by a sol-gel method comprising the steps of: dissolving indiluted nitric acid Y₂O₃ and Lu₂O₃, CeO₂ and/or TbO₂, and oxide ornitrate of Zr and/or Sm; adding an alcohol and a tetraethylorthosilicate to the obtained solution and then mixing them completely;adding diluted aqueous ammonia to the mixture to obtain a gel; andheating the gel at a temperature of 1,400 to 1,600° C.

JP-3,290,497B discloses a stimulable phosphor represented by theformula: Y_(2-x)Ln_(x)SiO₅●yM:zAc in which Ln is at least one rare earthelement selected from the group consisting of Y, Gd and Lu; Ac is atleast one element selected from the group consisting of Eu, Ce, Sm andZr; M is at least one element selected from the group consisting of Aland Mg; and x, y and z are numbers satisfying the conditions of 0<x≦2,0<y≦1.0 and 0<z≦0.1, respectively. The disclosed phosphor is prepared bya solid phase reaction method which comprises the steps of: mixing Y₂O₃and/or Lu₂O₃, SiO₂ and an oxide of Ac; adding AlF₃ and/or MgF₂ to themixture, and then mixing them again; and firing the obtained mixture.

IEEE Transaction on Nuclear Science, vol. 47(2000), No. 6, pp. 1781discloses a process for preparation of lutetium silicate phosphor. Theprocess comprises the steps of reacting lutetium metal with isopropanolto prepare a alkoxide and firing the alkoxide to obtain the lutetiumsilicate. According to this process, a phosphor giving high performanceswhen used as a scintillator can be obtained by firing at 1,200° C.However, it is necessary in the process to use mercury, which is anundesirable substance from the environmental viewpoint.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a new process forpreparation of a rare earth silicate phosphor. In particular, theinvention provides a process for preparation of a rare earth silicatephosphor efficiently absorbing radiation and giving strong simulatedemission.

The present applicant has studied a process for preparation of a rareearth silicate, particularly silicate of a heavy rare earth such aslutetium. As a result, the applicant has found a new process by whichthe rare earth silicate phosphor can be easily prepared.

As is described above, in the conventional process, the rare earthsilicate phosphor having good emission property is prepared by a processcomprising melting or firing the starting materials at a very hightemperature. In contrast, according to the new process, the rare earthsilicate phosphor can be easily prepared at a relatively low temperaturewithout using an unusual material. The thus-prepared phosphor ischemically stable and moisture-proof, and has excellent emissionproperty.

The invention, in the first place, resides in a process for preparationof a phosphor represented by the formula (I):Lu_(x)Y_(y)Gd_(z)SiO_(p):aA,bL   (I)in which A is at least one element selected from the group consisting ofCe, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb; L is at one elementselected from the group consisting of Zr, Nb, Hf, Ta, Sn, Sm, Tm and Yb,provided that L differs from A; x, y and z are numbers satisfying theconditions of 0≦x, 0≦y, 0≦z and 1.5≦x+y+Z≦2.2; p is a number toneutralize the phosphor in regard to electric charge thereof, a is anumber satisfying the condition of 2×10⁻⁵<a<6×10⁻², and b is a numbersatisfying the condition of 0≦b<1×10⁻²;which comprises at least the steps of:

(1) Heating a Rare Earth Carboxylate Represented by the Formula (II):(R¹—COO)₃M●MH₂O   (II)in which M is at least one rare earth element selected from the groupconsisting of Lu, Y and Gd; R¹ is an aliphatic hydrocarbon group having1 to 4 carbon atoms which may have either a substituent or nosubstituent; and m is a number satisfying the condition of 0≦m≦4;

with an alkoxyalcohol represented by the formula (III), to obtain asolution:R²—O—(CH₂)_(n)OH   (III)in which R² is an aliphatic hydrocarbon group having 1 to 4 carbon atomsor a substituted aliphatic hydrocarbon group having 3 to 6 carbon atoms;and n is 2 or 3;

(2) Adding the Obtained Solution a Silicon Alkoxide Represented by theFormula (IV):Si(OR³)₄   (IV)in which R³ is an aliphatic hydrocarbon group having 1 to 4 carbon atomsand a compound containing the element represented by A, and if requireda compound containing the element represented by L, to prepare amixture; and

(3) Subjecting the Prepared Mixture to Thermal Decomposition.

The invention, in the second place, resides in a process for preparationof a phosphor represented by the formula (I):Lu_(x)Y_(y)Gd_(z)SiO_(p):aA,bL   (I)in which A is at least one element selected from the group consisting ofCe, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb; L is at one elementselected from the group consisting of Zr, Nb, Hf, Ta, Sn, Sm, Tm and Yb,provided that L differs from A; x, y and z are numbers satisfying theconditions of 0≦x, 0≦y, 0≦z and 1.5≦x+y+Z≦2.2; p is a number toneutralize the phosphor in regard to electric charge thereof, a is anumber satisfying the condition of 2×10⁻⁵<a<6×10⁻², and b is a numbersatisfying the condition of 0≦b<1×10¹⁰⁻²;which comprises at least the steps of:

(1) Heating a Rare Earth Carboxylate Represented by the Formula (II):(R¹—COO)₃M●mH₂O   (II)in which M is at least one rare earth element selected from the groupconsisting of Lu, Y and Gd; R¹ is an aliphatic hydrocarbon group having1 to 4 carbon atoms which may have either a substituent or nosubstituent; and m is a number satisfying the condition of 0≦m≦4;

with an alkoxyalcohol represented by the formula (III), to give asolution:R²—O—(CH₂)_(n)OH   (III)in which R² is an aliphatic hydrocarbon group having 1 to 4 carbon atomsor a substituted aliphatic hydrocarbon group having 3 to 6 carbon atoms;and n is 2 or 3;

(2) Adding to the Obtained Solution a Silicon Alkoxide Represented bythe Formula (IV):Si(OR³)₄   (IV)in which R³ is an aliphatic hydrocarbon group having 1 to 4 carbon atoms

and a compound containing the element represented by A, and if requireda compound containing the element represented by L, to prepare amixture;

(3) Bringing Water into Contact with the Prepared Mixture with Water toPrepare a Gel; and

(4) Subjecting the Prepared Gel to Thermal Decomposition.

DETAILED DESCRIPTION OF THE INVENTION

In the first or second process of the invention, R¹ in the formula (II)preferably is methyl.

The alkoxyalcohol represented by the formula (III) preferably is atleast one compound selected from the group consisting of2-methoxyethanol, 2-ethoxyethanol, 1-methoxy-2-propanol,1-ethoxy-2-propanol and 3-ethoxy-1-propanol.

The group R³ in the formula (IV) preferably is ethyl.

The phosphor prepared by the first or second process of the invention ispreferably represented by the formula (V):Lu_(x)SiO_(p):aA′,bL′  (V)in which A′ is at least one element selected from the group consistingof Ce and Tb; L′ is at least one element selected from the groupconsisting of Zr, Hf, Sm and Yb, provided that L′ differs from A′; x isa number satisfying the condition of 1.5≦x≦2.2; p is a number toneutralize the phosphor in regard to electric charge thereof, and a is anumber satisfying the condition of 2×10⁻⁵<a<6×10⁻², and b is a numbersatisfying the condition of 0≦b≦1×10⁻².

The first process of the invention preferably comprises the steps of:

(1) heating an acetate of Lu, Y and/or Gd together with 2-methoxyethanoland/or 2-ethoxyethanol, to obtain a solution;

(2) adding to the obtained solution tetraethoxy-silane and a compoundcontaining the element represented by A, and a compound containing theelement represented by L, to prepare a mixture; and

(3) subjecting the prepared mixture to thermal decomposition.

The second process of the invention preferably comprises the steps of:

(1) heating an acetate of Lu, Y and/or Gd together with 2-methoxyethanoland/or 2-ethoxyethanol, to obtain a solution;

(2) adding to the obtained solution tetraethoxy-silane and a compoundcontaining the element represented by A, and if required a compoundcontaining the element represented by L, to prepare a mixture; and

(3) bringing water into contact with the prepared mixture with water togive a gel; and

(4) subjecting the given gel to thermal decomposition.

The processes of the invention are described below in detail.

[1] Step of Heating

A rare earth carboxylate and an alkoxyalcohol are heated together.

The rare earth carboxylate is represented by the formula (II):(R¹—COO)₃M●MH₂O   (II)in which M is at least one rare earth element selected from the groupconsisting of Lu, Y and Gd; R¹ is an aliphatic hydrocarbon group having1 to 4 carbon atoms which may be either substituted or non-substituted;and m is a number satisfying the condition of 0≦m≦4.

Examples of the rare earth carboxylates include lutetium acetate,yttrium acetate, gadolinium acetate, lutetium methoxyacetate, lutetiumhydroxyacetate, lutetium propionate, yttrium propionate, and gadoliniumpropionate. They may be either anhydrous salts or hydrates. Preferredare lutetium acetate, yttrium acetate, and gadolinium acetate in theform of anhydrous salts or hydrates. These rare earth carboxylates maybe used singly or in combination according to the composition of thedesired phosphor. The matrix of the phosphor is preferably constitutedof lutetium or a combination of lutetium and yttrium.

Prior to the preparation of the phosphor, the rare earth carboxylate maybe beforehand prepared in a reaction vessel. For example, a rare earthmetal, its oxide or its hydroxide is dissolved in an excess amount ofaqueous acetic acid, and then the excess acetic acid and water aredistilled off to obtain the rare earth acetate.

The alkoxyalcohol is represented by the formula (III):R²—O—(CH₂)_(n)OH   (III)in which R² is an aliphatic hydrocarbon group having 1 to 4 carbon atomsor a substituted aliphatic hydrocarbon group having 3 to 6 carbon atoms;and n is a number of 2 or 3.

Examples of the alkoxyalcohols include 2-methoxy-ethanol,2-ethoxyethanol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol,3-ethoxy-l-propanol, 2-(2-ethoxyethoxy)-ethanol,2-(2-methoxyethoxy)ethanol, acetoxyethanol, 2-isopropoxyethanol, and2-butoxyethanol. Preferred are 2-methoxyethanol, 2-ethoxyethanol,1-methoxy-2-propanol, 1-ethoxy-2-propanol and 3-ethoxy-l-propanol. Thesealkoxy-alcohols may be used singly or in combination. Further, they maybe used in the form of mixture with a compatible solvent such asdioxane, acetone, dimethylformamide or dimethylsulfoxide.

The rare earth carboxylate is dispersed in the alkoxyalcohol, and thenheated. The amounts of the compounds are determined according to thecomposition of the desired phosphor, but the total amount of rare earthcarboxylate is generally in the range of 0.01 to 1 mol, preferably inthe range of 0.05 to 0.5 mol based on one liter of the alkoxyalcohol.The temperature and period of time for heating are also determinedaccording to the amounts and characters of the compounds, but thecompounds are generally heated at a temperature of 100 to 150° C. forapprox. 3 to 8 hours. After heating, the obtained solution may becondensed by distillation under reduced pressure.

[2] Step of Mixing

To the obtained solution (reaction mixture), a silicon alkoxide and oneor more compounds of activator components are added and mixed. Thesilicon alkoxide is represented by the formula (IV)Si (OR³)₄   (IV)in which R³ is an aliphatic hydrocarbon group having 1 to 4 carbonatoms.

Examples of the silicon alkoxides (alkoxysilanes) includetetramethoxysilane, tetraethoxysilane, tetraisopropoxysilane,tetra-n-propoxysilane and tetraisobutoxysilane. Tetraethoxysilane isparticularly preferred.

The silicon alkoxide is added generally in an amount of 45 to 67 mol. %based on the amount of rare earth in the rare earth carboxylate.

The compound of activator component A is a water-soluble salt of Ce, Pr,Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and/or Yb. Examples of preferredcomponents A include Ce and Tb. Most preferred is Ce. The compound ofactivator component B is a water-soluble salt of Zr, Nb, Hf, Ta, Sn, Sm,Tm, and/or Yb. Examples of a preferred components L include Zr and Sm.These compounds can be, for instance, salts, chelate compounds, oralkoxide compounds. Nitrates thereof are preferred. Zr, Hf, Nb, and/orTa all of which belong to Group IV or V of the Periodic Table arepreferably used as alkoxide compounds such as ethoxy compounds orisopropoxy compounds.

If the rare earth activated rare earth silicate phosphor is employed asa scintillator utilizing the instant emission only, the phosphor maycontain only the activator component A. However, if the rare earthactivated rare earth silicate phosphor is employed as a stimulablephosphor, the phosphor preferably contains the activator components Aand B in combination. Preferred are a combination of Ce and Sm, and acombination of Ce and Zr.

The activator compound for the activator component A is added in anamount of 0.0005 to 3 mol. %, preferably 0.002 to 0.3 mol. % in terms ofthe activating element based on the total amount of rare earth elements(Lu, Y, Gd) (which constitute the phosphor matrix) contained in the rareearth carboxylate. If a compound for the activator component L is added,the compound is added in an amount of not more than 2 mol. %.

The silicon alkoxide and the activator compound(s) are preferablybeforehand dissolved in a solvent such as isopropyl alcohol, and thenadded and mixed in the solution obtained by the heating step. Further,after they are added, the resulting solution is preferably heated againto give a homogeneous solution.

[3] Step of Gelation

Thus-obtained mixture is then brought into contact with water or watervapor to prepare a gel.

In the present invention, it is not always necessary to perform the stepof gelation. After the silicon alkoxide and the activator compound areadded to the reaction mixture obtained by the heating step, the solventmay be distilled off, if needed, and then the resulting mixture may bedirectly subjected to the step of thermal decomposition (the firstprocess of the invention). However, prior to the step of thermaldecomposition, the mixture is preferably converted into a gel (thesecond process of the invention).

For preparing the gel, water diluted with a water-compatible solvent maybe added to the mixture, or otherwise the mixture may be exposed tohumid air. The solvent of the mixture is preferably removed bydistillation before the gelation.

[4] Step of Thermal Decomposition

The obtained gel is then subjected to thermal decomposed.

The gel is placed in a heat-resistant container such as an aluminacrucible, a platinum crucible or a quartz boat, and then fired in anelectric furnace. The firing temperature is generally in the range of800 to 1,700° C. The period of time for firing is normally in the rangeof 1 to 18 hours, preferably in the range of 2 to 12 hours, althoughdepending upon various conditions such as the amount of the gel, thefiring temperature and the temperature at which the fired gel is takenout of the furnace. The gel may be fired under not only air atmosphereor nitrogen gas atmosphere but also an atmosphere in the presence ofcarbon, both water vapor and hydrogen gas or both carbon monoxide andcarbon dioxide. The firing may be carried out once, but the gel may befired twice or more with the temperature and the atmosphere changed.

Prior to the firing, the gel may be crushed into particles. Further, inorder to perform the firing efficiently, a small amount of fluoride(e.g., NH₄F, AlF₃, MgF₂) may be added as a flux to the gel.

Organic components and water in the gel are decomposed and/or removed bythe firing, and thus the gel is converted into a silicate of complexoxide.

The obtained product may be, if needed, subjected to various treatments(such as crushing and sieving) that are generally carried out in thepreparation of phosphors. Further, for ensuring the high emissionproperty, the product may be annealed. In that case, the temperaturerange for annealing must be controlled so that the resultant phosphorparticles may not sinter.

Thus, a rare earth silicate phosphor represented by the formula (I) canbe prepared. In consideration of radiation-absorption efficiency andemission intensity, the phosphor of the invention preferably comprises amatrix mainly containing lutetium as the rare earth matrix component.Particularly preferably, the phosphor of the invention is represented bythe formula (V).Lu_(x)SiO_(p):aA′,bL′  (V)in which A′ is at least one element selected from the group consistingof Ce and Tb; L′ is at least one element selected from the groupconsisting of Zr, Hf, Sm and Yb, provided that L′ differs from A′; x isa number satisfying the condition of 1.5≦x≦2.2; p is a number toneutralize the phosphor in regard to electric charge thereof, and a is anumber satisfying the condition of 2×10⁻⁵<a<6×10⁻², and b is a numbersatisfying the condition of 0≦b<1×10⁻².

The crystal structure of the obtained phosphor can be assigned accordingto the X-ray diffraction method. For example, the silicate phosphormainly comprising lutetium gives a diffraction pattern of JCPDS card No.410239. The phosphor of the invention may have a complex structureconsisting of crystalline portions and amorphous portions.

The phosphor particles have an average size of generally 0.05 to 20 μm,preferably 0.5 to 10 μm. The phosphor particle may be a single crystalor a secondary particle such as a polycrystal or aggregate consisting ofcrystallites having sizes of 10 to 500 nm.

The rare earth silicate phosphor thus prepared according to theinvention gives stimulated emission when exposed to electromagnetic wavein a wide wavelength region ranging UV light to radiation, andaccordingly it can be utilized to various known uses. In particular, thephosphor can be advantageously used either in the aforementionedradiation image recording and reproducing method (in which a radiationimage storage panel is alone used) or in the radiation image formingmethod (in which a radiation image storage panel is used in combinationwith a phosphor screen, which may be unified with the storage panel).

A radiation image storage panel comprising the phosphor of the inventionis described below.

The radiation image storage panel at least has a stimulable phosphorlayer containing the stimulable phosphor of the invention by whichradiation energy is absorbed and stored. The phosphor layer may be aself-supporting sheet, but is normally provided on a support. On thephosphor layer, a protective film is preferably provided. For increasingsharpness of the resultant image, the phosphor layer and/or theprotective film may contain a colorant which does not absorb thestimulated emission but the stimulating rays, or otherwise anintermediate layer containing that colorant may be provided. Otherintermediate layers such as a light-reflecting layer and an adhesivelayer may be provided, if needed.

The radiation image storage panel may have a radiation-absorbingphosphor layer as well as the stimulable phosphor layer. Theradiation-absorbing phosphor layer is placed next to the stimulablephosphor layer, and contains a phosphor which imagewise absorbsradiation and instantly emits ultraviolet or visible light. In thatcase, the stimulable phosphor of the invention, which is contained inthe stimulable phosphor layer, absorbs the light emitted by theradiation-absorbing phosphor layer and gives stimulated emission whenexposed to stimulating rays of secondary excitation. These phosphorlayers are formed in a pair. Also in the storage panel having theradiation-absorbing phosphor layer, the above-described support,protective film and intermediate layers can be provided.

[Support]

The support of the storage panel may be made of any material havingsmooth plane. Examples of the supports include a polymer sheet, a plateof metal such as aluminum, a ceramic plate, and a glass plate. Thesupport may be transparent, or may contain light-reflecting substances(e.g., particles of alumina, titanium dioxide or barium sulfate) thatreflect the stimulating light or stimulated emission. Further, it maycontain voids or light-absorbing substances (e.g., carbon black) thatabsorb stimulating rays or stimulated emission.

Examples of the polymer sheets include sheets of various resins such aspolyethylene terephthalate, polyethylene naphthalate, aramide resin andpolyimide resin. The polymer sheet has a thickness of preferably 50 μmto 1 mm, more preferably 120 to 350 μm. The polymer sheet may beprovided on another substrate such as a sheet of carbon fiber oraluminum.

[Protective Film]

On the surface of the phosphor layer, a protective film may provided toprotect the layer chemically and physically. The protective film ispreferably transparent enough not to affect the stimulating rays and thestimulated emission, and is also preferably enough chemically stable andphysically strong to keep the phosphor layer from chemical deteriorationor physical shock. The protective film can be formed by various method,for example, by coating the phosphor layer with a solution oftransparent organic polymer (e.g., cellulose derivatives,polymethylmethacrylate, fluorine resin soluble in organic solvents)dissolved in an appropriate solvent, by fixing a beforehand preparedtransparent sheet (e.g., a glass plate, a film of organic polymer suchas polyethylene-terephthalate) with adhesive, or by depositing inorganicmaterials on the phosphor layer. Various additives such as a slippingagent (e.g., powders of perfluoroolefin resin and silicone resin) and acrosslinking agent (e.g., polyisocyanate) may be dispersed in theprotective film.

The protective film is preferably made to be light-scattering in acertain degree so as to increase sharpness of the resultant radiationimage. The protective film generally gives a scattering length of 5 to80 μm, preferably 10 to 70 μm, at the main wavelength of the stimulatedemission from the stimulable phosphor. The light-scattering protectivefilm can be formed by dispersing light-scattering fine particles in theaforementioned material for the film. The light-scattering fineparticles have a refractive index of preferably 1.6 or more, morepreferably 1.9 or more, and sizes of preferably 0.1 to 1.0 μm, morepreferably 0.1 to 0.5 μm. Examples of the light-scattering fineparticles include fine particles of benzoguanamine resin,melamine-formaldehyde condensed resin, zinc oxide, zinc sulfide, andtitanium oxide.

For enhancing the resistance to stain, a fluororesin layer may beprovided on the protective film. The fluororesin layer can be form bycoating the surface of the protective film with a solution in which afluororesin is dissolved (or dispersed) in an organic solvent, anddrying the applied solution. The fluororesin may be used singly, but amixture of the fluororesin and a film-forming resin is normallyemployed. In the mixture, an oligomer having polysiloxane structure orperfluoroalkyl group can be further added. In the fluororesin layer,fine particle filler may be incorporated to reduce blotches caused byinterference and to improve the quality of the resultant image. Thethickness of the fluororesin layer is generally in the range of 0.5 to20 μm. For forming the fluororesin layer, additives such as acrosslinking agent, a film-hardening agent and an anti-yellowing agentcan be used. In particular, the cross-linking agent is advantageouslyemployed to improve durability of the fluororesin layer.

The protective film or the fluororesin layer has a coefficient ofmaximum friction in the range of preferably 0.18 or less, morepreferably 0.12 or less, so that the storage panel can be easily handledwhen installed in and taken out of a cassette for recording. The averagesurface roughness is also preferably in the range of 0.05 to 0.5 μm,more preferably in the range of 0.1 to 0.3 μm. For example, a greatnumber of very small convexes or concaves may be formed by embossing onthe surface of the protective film or the fluororesin layer.

It is also preferred that anti-static materials be incorporated in theprotective film or other layers so as to prevent the storage panel fromelectrification. If the panel is electrified, static marks are oftenmade by discharge. In recording the radiation image, the electric chargeon the panel is preferably removed before installation in the cassette.

The protective film has a thickness of generally approx. 1 to 20 μm,preferably 3 to 15 μm.

[Phosphor Layer]

The phosphor layer comprising the phosphor of the invention generally isa layer in which particles of the phosphor are dispersed in a binder.The phosphor layer, however, may comprise agglomerate of the phosphorwithout binder, or otherwise may comprise the agglomerated phosphorsoaked with a polymer. The phosphor layer comprising a binder and thephosphor particles dispersed therein can be prepared in the followingmanner.

First, the phosphor particles and a binder are well mixed in an organicsolvent to prepare a coating liquid in which the particles arehomogeneously dispersed in the organic solution of binder. The bindercan be optionally selected from known various resins conventionally usedas a binder. Examples of the binder include gum arabic, dextran,polyvinyl acetate, hydroxyethyl cellulose, polymethyl methacrylate,polybutyl methacrylate, polyurethane and polyvinyl alcohol. The ratiobetween the binder and the phosphor in the liquid depends on thecharacteristics of the phosphor and the aimed property of the panel, butis generally in the range of 1 to 0.01 (binder/phosphor, by weight). Thecoating liquid may contain a dispersing agent to assist the phosphorparticles in dispersing, and also contain other additives such as aplasticizer for increasing bonding between the binder and the phosphorparticles, an anti-yellowing agent for preventing the phosphor layerfrom yellowing, a film-hardening agent and a crosslinking agent.

Thus prepared coating liquid is evenly applied on the support by knowncoating means such as doctor blade, roll coater and knife coater, andthen dried to form a stimulable phosphor layer. The phosphor layer maybe formed by other procedures, namely, applying the above coating liquidonto a temporary support (e.g., glass plate, metal plate, plasticsheet), drying the applied liquid to form a phosphor sheet, peeling offthe phosphor sheet, and then providing the phosphor sheet with adhesiveor by pressing onto the support.

The phosphor layer may be a single layer or may consist of two or moresub-layers. For example, the phosphor layer preferably consists of twosub-layers comprising phosphors having the same composition butdifferent sizes to prevent light-scattering in the phosphor layer. Theabove-mentioned procedure can be also used to prepare a radiation imagestorage panel in which the radiation-absorbing phosphor layer and thestimulable phosphor layer are separately provided so that the stimulablephosphor layer can absorb ultraviolet light emitted from theradiation-absorbing phosphor layer. In that case, the phosphor layerscan be successively formed.

The thickness of the stimulable phosphor layer depends on the structureof the storage panel. In a normal storage panel in which the stimulablephosphor layer absorbs radiation, the thickness is generally in therange of 50 to 500 μm, preferably in the range of 100 to 300 μm. In astorage panel comprising both of the radiation-absorbing phosphor layerand the stimulable phosphor layer, it is generally in the range of 1 to50 μm, preferably in the range of 5 to 20 μm.

[Instantly Emitting Phosphor]

In the radiation image storage panel in which the radiation-absorbingphosphor layer and the stimulable phosphor layer are separately providedso that the stimulable phosphor layer can absorb ultraviolet lightemitted by the radiation-absorbing phosphor layer, theradiation-absorbing phosphor layer contains an instantly emittingphosphor which comprises an element having an atomic number of 37 ormore as a main component of the matrix and which has a higher truedensity than the phosphor of the invention. Examples of the instantlyemitting phosphor include LnTaO₄: (Nb, Gd, Tm), LnAlO₃:Ce and Lu₂O₃:Gd.The thickness of the instantly emitting phosphor layer(radiation-absorbing phosphor layer) is generally in the range of 50 to500 μm, preferably in the range of 100 to 300 μm.

The instantly emitting phosphor preferably gives instant emission whosewavelength range covers 70% or more of the wavelength range of theprimary excitation for the stimulable phosphor. Here, the term“wavelength range” means a wavelength region in which the intensity is10% or more based on the intensity at the maximum peak of the instantemission spectrum or the primary excitation spectrum.

The phosphor layer may comprise a binder and the phosphor particlesdispersed therein, may comprise agglomerate of the phosphor withoutbinder, or otherwise may comprise the agglomerated phosphor soaked witha polymer.

[Partition]

In the radiation-absorbing phosphor layer and/or the stimulable phosphorlayer, a partition may be optionally provided to reduce scattering andto increase sharpness of the resultant image. The partition is placed sothat the plane of the layer may be divided into small sections. Sincethe phosphor layer is relatively thick, the partition effectivelyprevents the emission from diffusing. The partition may be in a desiredshape such as stripes or grating. It is also possible to enclose thephosphor with the partition in any shape such as circle or hexagon. Bothof the top and the bottom of the partition may appear on the surfaces ofthe layer, or otherwise one or both of them may be buried in the layer.

The partition can be provided, for example, in the following manner.First, a sheet or plate of metal (e.g., aluminum, titanium, stainlesssteel), ceramics (e.g., aluminum oxide, aluminum silicate) or organicpolymer material (photo-sensitive resin) is subjected to a properetching treatment, to prepare a honeycomb sheet having many dimples(holes) or porosities. The above-described phosphor layer is then placedon the prepared honeycomb sheet, and heated and pressed so that thesheet may be pushed into the phosphor layer. As a result, a phosphorlayer comprising a honeycomb partition can be thus obtained. Otherwise,many thin phosphor sheets comprising a binder and the phosphor particlesdispersed therein are beforehand prepared. Independently, many thinpartition sheets made of polymer material are also beforehand prepared.The phosphor sheets and the partition sheets are alternatively piled upto prepared a layered composition, and then the composition isperpendicularly sliced off. The obtained slice is a phosphor sheetcomprising a striped partition. The partition can containlow-light-absorbing fine particles such as aluminum oxide or titaniumoxide, or may contain colorant that selectively absorbs the instantemission from the radiation-absorbing phosphor. The partition may bemade of materials of the phosphor layer. (In that case, however, theratio of binder/phosphor and the size of phosphor particles aredifferent from those for forming the phosphor layer.)

The radiation image storage panel may comprise other layers such as aselectively-reflecting layer and a diffusing-reflecting layer.

[Colorant]

For increasing sharpness of the radiation image, at least one layer inthe radiation image storage panel may be colored with a colorant whichabsorbs the (instant) emission from the instantly emitting phosphorand/or the stimulating rays of secondary excitation [which are appliedto the stimulable phosphor layer in reading out a latent (storedradiation) image] or, in some cases, with a colorant which partlyabsorbs the (stimulated) emission from the stimulable phosphor.Practically, the phosphor layer, the protective film or an intermediatelayer such as an undercoating layer is colored with the colorantabsorbing the emission from the instantly emitting phosphor and/or thestimulating rays of secondary excitation. One of the above layers may besingly colored, or they may be colored partly in any combination. If theradiation image stored in the panel is read out with a photomultipliertube in the point-detecting system described after, it is preferred forthe colorant not to absorb the emission from the stimulable phosphor.

For the radiation image storage panel in which the stimulable phosphorlayer directly absorbs radiation, the colorant is selected according tothe properties of the phosphor so that the stimulating rays of secondaryexcitation may not be transmitted but the stimulated emission may be.The colorant used in many normal cases does not transmit red light butblue to green light.

For the radiation image storage panel comprising the instantly emittingphosphor (which absorbs radiation and instantly emits light) and thestimulable phosphor (which absorbs the light emitted from the instantlyemitting phosphor and gives stimulated emission when exposed to thestimulating rays of secondary excitation) in combination, the colorantis selected so that it may absorb both of the instant emission and thestimulating rays of secondary excitation. For example, in the case wherethe instantly emitting phosphor emits green light and the stimulablephosphor absorbs it and gives stimulated emission in the red wavelengthregion when exposed to near infrared rays of secondary excitation, thecolorant preferably does not absorb red light but green light and nearinfrared rays (if the radiation image is read out in the point-detectingsystem). Two or more colorants may be used in combination.

The colorant suitable for the above case is a red colorant. Examples ofthe red colorant include inorganic pigments such as cadmium red, rediron oxide and molybdenum red. These red colorants, however, scarcelyabsorb near infrared light, and hence are preferably used in combinationwith near infrared-absorbing substances such as cyanine dye, indoanilinedye and squarilium dye.

In the case where the instantly emitting phosphor emits near ultravioletlight and the stimulable phosphor absorbs it and gives stimulatedemission in the blue to green wavelength region when exposed to redlight of secondary excitation, the colorant preferably does not absorbblue to green light but near ultraviolet and red light (if the radiationimage is read out in the point-detecting system). The colorant suitablefor that case is a blue or green colorant.

If the radiation image is read out not in the point-detecting systememploying a photomultiplier tube but in a line-detecting systememploying a line sensor, the instantly emitting phosphor layer and theintermediate layer such as the undercoating layer are preferably coloredwith a colorant which absorbs the instant emission from the instantlyemitting phosphor, the simulating rays of secondary excitation for thestimulable phosphor and/or the stimulated emission from the stimulablephosphor. If the stimulated emission diffuses out of the area havingbeen exposed to the simulating rays of secondary excitation, theresultant image is liable to get blurred. In the case where theradiation-absorbing phosphor emits green light and the stimulablephosphor absorbs it and gives stimulated emission in the red wavelengthregion when exposed to near infrared rays of secondary excitation, thecolorant preferably absorbs green, red and/or near infrared light. Inother words, a red, blue, green or gray colorant absorbing near infraredlight is preferred. The above-described red colorants are usable.Examples of the blue or green colorant absorbing near infrared lightinclude titanyl phthalocyanine TiO—Pc (San-yo Dye Co., Ltd.). Theabove-described blue or green colorants may be used in combination withinfrared-absorbing substances. Examples of the gray colorant includecarbon black and Cu—Fe—Mn oxide.

The present invention is further described by the following examples.

EXAMPLE 1

Preparation of Lu₂SiO:0.001 Ce phosphor

21.2 g of lutetium acetate tetrahydrate was dispersed in 500 ml ofethoxyethanol, and then the prepared dispersion was refluxed at 136° C.for 7 hours to remove approx. 170 ml of distillate. To the obtained paleyellow transparent liquid, a mixture of 5.86 ml of tetraethoxysilane and11 mg of cerium nitrate hexahydrate dissolved in 50 ml of isopropanolwas added. The reaction liquid was refluxed for 2 hours to mix well, andthen the solvent was distilled off at 90° C. under reduced pressure(approx. 10 mmHg) to obtain viscous yellow liquid of precursor. Theliquid of precursor was spread in a glass laboratory dish, and exposedto atmospheric humidity for approx. 10 hours. Thus, transparent yellowglassy gel was prepared.

The gel was crushed, and the obtained particles were made to have evensizes. After stuffed in an alumina crucible, the gel particles wereplaced in the core of a muffle furnace and then fired at 1,000° C. for 1hour under air atmosphere. Further, the firing was successively carriedout again at the temperature set forth in Table 1 for 4 hours underslightly reductive atmosphere in the presence of a little carbon. Thus,Lu₂SiO₅:0.001 Ce phosphors A to D were prepared according to theinvention.

Evaluation of Phosphor (1)

The crystal structures of the obtained phosphors A to D were analyzed bythe X-ray diffraction method.

In addition, the intensity of stimulated emission given by each phosphorwas evaluated in the following manner. Each of the phosphors A to D wasstuffed in a dimple (250 μm) of a holder for measurement, and exposed toX-rays of 40 kVp in the amount of 4.6 J/m² (200 mR). After 10 seconds,each phosphor was exposed to He—Ne laser (wavelength: 633 nm) andthereby given stimulated emission was detected through an optical filter(B-410) with a photomultiplier tube. The amount of emission wasintegrated for 10 seconds since the emission began to come out, andreduced to a relative value for evaluating the intensity of stimulatedemission.

The results are set forth in Table 1. TABLE 1 Temp. of Stimulatedemission Phosphor Precursor 2nd firing Crystal λ_(max) Intensity Ex. 1-AGel 1,100° C. LSO* 400 nm 50 Ex. 1-B Gel 1,200° C. LSO* 400 nm 70 Ex.1-C Gel 1,400° C. LSO* 400 nm 98 Ex. 1-D Gel 1,600° C. LSO* 400 nm 101*LSO: The crystal gave a diffraction pattern of JCPDS card No. 410239.

The results shown in Table 1 indicate that, according to the invention,the aimed lutetium orthosilicate (LSO) salt can be prepared by firingeven at 1,100° C. Namely, the present invention makes it possible toobtain a phosphor giving strong stimulated emission by firing even at arelatively low temperature.

EXAMPLE 2

Preparation of Lu₂SiO₅:0.001 Ce, A′″ phosphor (A′″: co-activator)

21.2 g of lutetium acetate tetrahydrate was dispersed in 500 ml ofethoxyethanol, and then the prepared dispersion was refluxed at 136° C.for 7 hours to remove approx. 170 ml of distillate. To the obtained paleyellow transparent liquid, a mixture of 5.86 ml of tetraethoxysilane, 7mg of triethoxy cerium and each co-activator shown in Table 2 dissolvedin 50 ml of isopropanol was added. The reaction liquid was refluxed for2 hours to mix well, and then the solvent was distilled off at 90° C.under reduced pressure (approx. 10 mmHg) to obtain viscous yellow liquidof precursor. The liquid of precursor was spread in a glass laboratorydish, and exposed to atmospheric humidity for approx. 10 hours. Thus,transparent yellow glassy gel was prepared.

The gel was crushed, and the obtained particles were made to have evensizes. After stuffed in an alumina crucible, the gel particles wereplaced in the core of a muffle furnace and then fired at 1,000° C. for 1hour under air atmosphere. Further, the firing was successively carriedout again at 1,400° C. for 4 hours under slightly reductive atmospherein the presence of a little carbon. Thus, the phosphors E to I wereprepared according to the invention. TABLE 2 Amount of co-activatorPhosphor Co-activator (mole per 1 mole of Lu) E Tetraisopropoxyzirconium 5 × 10⁻⁴ F Tetraisopropoxy zirconium 2.5 × 10⁻⁴   GTriisopropoxy samarium 5 × 10⁻⁴ H Triisopropoxy samarium 2.5 × 10⁻⁴   I— —

Evaluation of Phosphor (2)

The intensity of stimulated emission given by each of the preparedphosphors E to I was evaluated in the above-described manner.

The results are set forth in Table 3. TABLE 3 Intensity of PhosphorComposition stimulated emission E Lu₂SiO₅:0.001Ce, 0.001Zr 150 FLu₂SiO₅:0.001Ce, 0.0005Zr 120 G Lu₂SiO₅:0.001Ce, 0.001Sm 160 HLu₂SiO₅:0.001Ce, 0.0005Sm 130 I Lu₂SiO₅:0.001Ce 90

The results shown in Table 2 indicate that, according to the invention,the stimulated emission is enhanced by the co-activator Zr or Sm usedtogether with the activator Ce.

EXAMPLE 3

Preparation of Lu_(x)Y_(y)SiO₅: 0.001 Ce, 0.001 Sm phosphor

The procedures of Example 1 were repeated except that lutetium acetatetetrahydrate was partly or fully replaced with an equivalent amount ofyttrium acetate, that cerium nitrate hexahydrate was replaced with anequivalent amount of triethoxy cerium and triisopropoxy samarium as theco-activator and that the second firing was carried out at 1,400° C.Thus, the phosphors J to N were prepared according to the invention.

Evaluation of Phosphor (3)

The intensity of stimulated emission given by each of the preparedphosphors J to N was evaluated in the above-described manner.

The results are set forth in Table 4. TABLE 4 Intensity of PhosphorComposition stimulated emission J LuYSiO₅:0.001Ce, 0.001Sm 120 KLu_(1.5)Y_(0.5)SiO₅:0.001Ce, 0.001Sm 130 L Lu_(1.8)Y_(0.2)SiO₅:0.001Ce,0.001Sm 150 M Lu_(1.8)SiO_(4.7):0.0009Ce, 0.0009Sm 180 N Y₂SiO₅:0.001Ce,0.001Sm 72

The results shown in Table 4 indicate that, according to the invention,the stimulated emission is enhanced by increasing the ratio of Lu per Yin the phosphor matrix.

EXAMPLE 4 Adjustment of Firing Atmosphere

A gel prepared in the same manner as in Example 1 was pulverized. Thepulverized gel was adjusted to particle size adjustment. The particlesize adjusted pulverized gel was placed in an alumina crucible, and thecrucible was placed in the core of a muffle furnace and then fired at1,200° C. for 1 hour under air atmosphere. The crucible was transferredinto the core of a tube furnace and fired at 1,400° C. for 4 hours,under controlling the water vapor partial pressure and hydrogen pressureso as to control an equilibrium oxygen partial pressure in the core area(see the partial pressure set forth in Table 5).

Each of the phosphors O to S was stuffed in a dimple (250 μm) of aholder for measurement and exposed to X-rays of 40 kVp in the amount of4.6 J/m² (200 mR). The instant emission spectrum and its peak intensitygiven by each phosphor was measured.

The results are set forth in Table 5. TABLE 5 Instant EmissionEquilibrium Wavelength Intensity Phosphor O₂ pressure (nm) (relativevalue) O <10⁻¹⁴   400 20 P 10⁻¹¹ 400 120 Q 10⁻¹⁰ 400 250 R 10⁻⁹   400290 S 10⁻⁸   400 300

As is apparent from the data in Table 5, the process of the inventiongives rare earth activated rare earth silicate phosphors producinginstant emission of a high intensity by adjusting the firing atmosphere.

EXAMPLE 5

A radiation image storage panel comprising the phosphor E of Example 2was prepared in the following manner.

(Preparation of Coating Liquid for Phosphor Layer)

In a mixed solvent of methyl ethyl ketone-toluene (1:1), 356 g of thephosphor E of Example 2, 15.8 g of polyurethane resin (Desmorac 4125[trade name], available from Sumitomo Bayer Urethane Co., Ltd.) and 2.0g of Bisphenol A epoxy resin were added and mixed by means of propellermixer, to prepare a coating liquid for phosphor layer

(Preparation of Coating Liquid for Protective Film)

Into a mixed solvent of toluene-isopropanol (1:1), 70 g offluoroethylene-vinyl ether copolymer (Lumiflon LF504X [trade name],available from Asahi Glass Co., Ltd.), 25 g of isocyanate (DesmoduleZ4370 [trade name], available from Sumitomo Bayern Urethan Co., Ltd.), 5g of bisphenol A epoxy resin and 10 g of silicone resin powder (KMP-590[trade name], available from The Shin-Etsu Chemical Co., Ltd.; grainsize: 1 to 2 μm) were added, to prepare a coating liquid for protectivefilm.

(Formation of Phosphor Layer and Protective Film)

The above-prepared coating liquid for phosphor layer was applied onto apolyethylene terephthalate film beforehand provided with an undercoatinglayer, and dried at 100° C. for 15 minutes to form a stimulable phosphorlayer of 350 μm thickness. The coating liquid for protective film wasthen applied onto the phosphor layer, and dried at 120° C. for 30minutes, to form a protective film of 10 μm thickness. Thus, a radiationimage storage panel comprising a support, a phosphor layer and aprotective film was produced.

(Test for Detecting Radiation Image Pattern)

The prepared storage panel was exposed to 1 mR of a convergent beam ofX-rays emitted by an X-ray generator of 80 kVp. After 30 seconds, thepanel was scanned with a semiconductor laser bean (633 nm). Through aband-pass filter transmitting light in the wavelength range of 380 to500 nm, the stimulated emission was detected with a photo-multipliertube. As a result, it was confirmed that the signals obtained from thespot having been exposed to the X-rays were much stronger than thosefrom the area having not been exposed. This means the storage panel cangive a radiation image clearly enough to be practically used.

In the invention, a rare earth alkoxide preferably in the form of gel isthermally decomposed and thereby a rare earth silicate phosphor can beeasily prepared at a lower temperature than in a conventional processwithout using an unusual material. The thus-prepared phosphor,particularly the phosphor comprising lutetium, highly absorbs radiationand gives strong stimulated emission, and in addition is chemicallystable and moisture-proof. The phosphor prepared according to theinvention, therefore, can be advantageously used not only as astimulable phosphor in the radiation image recording and reproducingmethod or in the radiation image forming method but also as ascintillator or as a phosphor of a radiographic intensifying screen inthe radiography.

1-6. (canceled)
 7. A process for preparation of a stimulable phosphorrepresented by the formula (I):Lu_(x)Y_(y)Gd_(z)SiO_(p):aA, bL   (I) in which A is at least one elementselected from the group consisting of Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho,Er, Tm, and Yb; L is at one element selected from the group consistingof Zr, Nb, Hf, Ta, Sn, Sm, Tm and Yb, provided that L differs from A; x,y and z are numbers satisfying the conditions of 0≦x, 0≦y, 0≦z and1.5≦x+y+Z≦2.2; p is a number to neutralize the phosphor in regard toelectric charge thereof, a is a number satisfying the condition of2×10⁻⁵<a<6×10⁻², and b is a number satisfying the condition of0≦b<1×10⁻², in which the stimulable phosphor absorbs and stores aportion of energy of radiation or ultraviolet rays when it is exposed tothe radiation or ultraviolet rays, and emits stimulated light in avisible wavelength region when exposed to electromagnetic waves; whichcomprises the steps of: (1) heating a rare earth carboxylate representedby the formula (II)(R¹—COO)₃M●mH₂O   (II) in which M is at least one rare earth elementselected from the group consisting of Lu, Y and Gd; R¹ is an aliphatichydrocarbon group having 1 to 4 carbon atoms which has a substituent orno substituent; and m is a number satisfying the condition of 0≦m≦4;with an alkoxyalcohol represented by the formula (III), to obtain asolution:R²—O—(CH₂)_(n)OH   (III) in which R² is an aliphatic hydrocarbon grouphaving 1 to 4 carbon atoms or a substituted aliphatic hydrocarbon grouphaving 3 to 6 carbon atoms; and n is 2 or 3; (2) adding to the obtainedsolution a silicon alkoxide represented by the formula (IV):Si(OR³)₄   (IV) in which R³ is an aliphatic hydrocarbon group having 1to 4 carbon atoms and a compound containing the element represented byA, and if required a compound containing the element represented by L,to prepare a mixture; (3) bringing water into contact with the preparedmixture to give a gel; and (4) subjecting the given gel to thermaldecomposition under slightly reductive atmosphere.
 8. The process ofclaim 7, wherein R¹ in the formula (II) is methyl.
 9. The process ofclaim 7, wherein the alkoxy-alcohol represented by the formula (III) isat least one compound selected from the group consisting of2-methoxyethanol, 2-ethoxyethanol, 1-methoxy-2-propanol,1-ethoxy-2-propanol and 3-ethoxy-1-propanol.
 10. The process of claim 7,wherein R³ in the formula (IV) is ethyl.
 11. The process of claim 7,comprising the steps of (1) heating a rare earth carboxylate that is anacetate of at least one element selected from the group consisting ofLu, Y, and Gd with at least one alcohol that is an alkoxyalcoholselected from the group consisting of 2-methoxyethanol and2-ethoxyethanol, to obtain a solution; (2) adding to the obtainedsolution a silicon alkoxide that is tetraethoxysilane and a compoundcontaining the element represented by A, and if required a compoundcontaining the element represented by L, to prepare a mixture; and (3)bringing water into contact with the prepared mixture, to give a gel;and (4) subjecting the given gel to thermal decomposition.
 12. Theprocess of claim 7, wherein the phosphor is represented by the followingformula (V):Lu_(z)SiO_(p):aA′, bL′  (V) in which A is A′ and is at least one elementselected from the group consisting of Ce and Tb; L is L′ and is at leastone element selected from the group consisting of Zr, Hf, Sm and Yb,provided that L′ differs from A′; x is a number satisfying the conditionof 1.5≦x≦2.2; y is zero; z is zero; p is a number to neutralize thephosphor in regard to electric charge thereof, and a is a numbersatisfying the condition of 2×10⁻⁵<a<6×10⁻²; and b is a numbersatisfying the condition of 0≦b<1×10⁻².